Mesopic Illumination in Natural Environments: Implications for Myopia Research

This study aims to determine the duration of mesopic light conditions in natural environments.

Illuminance levels were measured at three outdoor locations (a city terrace, a forest and a shadowy park) during different seasons using a calibrated high-spec lux meter with a sensitivity threshold of 0.01 lux. Measurements were recorded from 1 h before sunset until complete darkness to capture the transition from photopic to mesopic (<40 lux) and scotopic (<0.1 lux) conditions. The duration and onset of mesopic light were analysed across locations and seasons to identify site-specific variations.

A gradual decline in illumination was seen from photopic light at 1 h before sunset down to mesopic and scotopic levels. Among the three measured locations, the shadowy park transitioned to mesopic conditions 15-20 min earlier than the terrace or forest. However, despite these differences in onset, the duration of the mesopic period remained consistent, lasting approximately 25-30 min across all locations during the different seasons.

In natural outdoor environments, mesopic light exposure during sunset represents a relatively brief and consistent transition period of approximately 25-30 min. These findings provide objective field-based baseline data on the temporal dynamics of mesopic illumination. While the present study did not assess refractive development or myopia status, the quantified environmental parameters may inform future research integrating wearable light sensors and refractive outcomes.